1. Field of the Invention
The present invention relates to a method for manufacturing an electronic component including an electrode layer including α-tungsten and also relates to such an electronic component and a surface acoustic wave filter.
2. Description of the Related Art
Conventionally, electrode layers for electronic components such as surface acoustic wave devices need to have low resistance. For example, electrode layers for interdigital transducers and reflectors used for surface acoustic wave devices need to have low resistance and also need to have high hardness. Furthermore, such electrode layers need to have low stress so as not to cause substrates to warp during layer formation.
Among metal films, a tungsten film has a bulk specific resistance of about 5 μΩ·cm, which is a small value, and has considerably high hardness. Thus, when the electrode layers for surface acoustic wave devices include tungsten, low insertion loss can be achieved. However, in the case of tungsten, it is known that layer stress, which causes substrate warping, and the change in specific resistance are large, depending on the pressure during layer formation.
In Japanese Unexamined Patent Application Publication No. 5-9721, hereinafter referred to as Related Document 1, the following technique is disclosed: a bias voltage is applied to a substrate and sputtering is performed to form an electrode layer including tungsten. In this technique, the layer stress can be controlled by applying the bias voltage while the specific resistance is prevented from increasing. Thus, a tungsten layer having a specific resistance of 11 μΩ·cm or less and a stress of 1 GPa or less, which are small values, can be formed by controlling the bias voltage V, the distance TS between a target and the substrate, and the pressure P during the layer formation.
On the other hand, in the Japanese Unexamined Patent Application Publication No. 5-263226, hereinafter referred to as Related Document 2, the following technique is disclosed: a tungsten layer is formed by a sputtering process using a mixed gas containing Ar and Xe. This technique uses the following phenomenon: the stress of a tungsten layer formed using Xe gas is different from that of another tungsten layer formed using an Ar gas. Thus, a tungsten layer having low specific resistance and low stress can be formed by controlling the mixing ratio of Xe and Ar gases within the following range:0.1≦Ar/(Ar+Xe)≦0.4
In the bias sputtering process disclosed in Related Document 1, a bias voltage must be applied to the substrate. Thus, there is a problem in that the sputtering apparatus is complicated and the degree of design freedom is reduced. Furthermore, layers cannot be formed with an ordinary sputtering apparatus.
In the sputtering process, disclosed in Document 2, using the mixed gas, the cost of forming layers is high because the sputtering gas is expensive.